Intra-urban variability of particulate matter (PM2.5 and PM10) and its relationship with optical properties of aerosols over Delhi, India

• Spatial variability of particulate matter was studied over Delhi.
• PM2.5 was ~ 10 times higher concentration compared to USEPA standard.
• Contribution of fine mode particle in inhalable particle was ~ 51%
• Large heterogeneity in PM2.5 was due to different emission sources

Highly time-resolved measurements of particulate matter (PM: PM2.5 and PM10) were made at three different sites across Delhi (CCRI: a highly traffic site, IMD: a less traffic site and IITM: an urban background site) from 1st December, 2011 to 30th June, 2013. Also, coarse mode (PM10–2.5) mass was estimated as the difference between PM10 and PM2.5. In addition, columnar aerosol optical properties such as aerosol optical depth (AOD) and Angstrom exponent (AE) were studied concurrently over IMD. The mean mass concentrations of PM2.5, PM10–2.5 and PM10 were 118.3 ± 81.7, 113.6 ± 70.4 and 232.1 ± 131.1 μg m− 3, respectively. Among the three sites, relatively higher mass concentrations of PM2.5 (~ 35% and 3%) were observed at CRRI compared to IMD and IITM.PM10 and PM10–2.5 were higher at these sites by ~ 31% and 19%; and 27% and 40%, respectively, compared to CRRI. Coefficients of divergence (COD) and correlation coefficients (r) were calculated between site pairs to assess the spatial and temporal heterogeneity of PM and moderate spatial divergence was found over the three sites. Traffic emission particles (PM2.5) exhibited high spatial heterogeneity as well. The mass concentrations of PM2.5 and PM10 were found to be higher during the night compared to the day. The mean PM2.5/PM10 ratio was ~ 51%, indicating generally equal amounts of coarse and fine mode PM in the Delhi urban atmosphere. AOD and PM2.5 were positively correlated and a negative correlation was observed between AE and PM10–2.5. PM2.5 particles were significantly correlated with AOD during post-monsoon and winter. Because of the lower vehicular emissions on weekends compared to weekdays, PM at CRRI, IMD, and IITM were separated by day of week and large heterogeneities were found. During weekdays, the mass concentrations of PM10 were ~ 4, 2, and 12% higher than on weekends. However, for PM2.5, weekend values were 5, 7, and 9% higher for CRRI, IMD and IITM, respectively.